The need to reduce data transmission costs necessitates increased satellite capacity. By the same token, increased user throughput to serve applications of multimedia type entails ever greater consumption of bandwidth.
Advances in satellite communication systems have responded to some of these requirements by offering high-throughput services, notably for Internet-related applications. These systems generally produce several tens of beams to cover wide geographical zones of interest, such as continents. Henceforth, new satellite systems are being designed to offer capacities for throughputs of greater than 100 Gb/s, therefore with a higher number of beams, more than one hundred for example. A consequence of these new systems with a large number of beams is that the service zones are sampled with smaller elementary spots and that ever larger antennas, as regards reflector size, are required on the satellite. Thus, for satellites developed with a short-term horizon, and operating in the 20-GHz frequency band, the reflectors have sizes of between 1.7 m to 2.6 m; for medium-term needs, the anticipated sizes lie between 3 m and 4 m, while the reflectors to be developed for long-term needs should exceed 5 m in diameter.
Furthermore, antenna systems are constructed according to an architecture of “one feed, one beam” type, stated otherwise an architecture associating a spot with a source (a source also being describable by the term radiating element), which makes it possible to configure the payload of a satellite in a fairly simple manner. A significant parameter in the design of the antenna system is that of the size of the sources employed. Conventionally, the latter are dimensioned in such a way as to obtain satisfactory antenna efficiency. Hence, the sources are generally dimensioned so that the illumination on the edges of the reflector lies between −7 dB and −15 dB with respect to the illumination at the centre of the reflector. The sources are thereafter installed in the focal plane side by side. With this configuration, a first source alongside a second source will generate a spot that is distant from the spot generated by the second source, so that the beams of a single antenna will illuminate the geographical zone only in a discontinuous manner. A second antenna is therefore necessary in order to plug the hole on an axis, and a third or indeed a fourth antenna depending on the mesh in order to fully complete the coverage with a set of beams ensuring the links with the EIRP (“Equivalent Isotropic Radiated Power”) and G/T levels required for the service quality envisaged.
Thus, having regard to these constraints, a conventional architecture relies on four antennas each operating in transmit/receive (RX/TX) mode. Indeed, having regard to the limited space on craft launchers, an architecture with eight antennas comprising four antennas in transmit mode (TX) and four antennas in receive mode (RX) would be very difficult to implement.
This manner of operation in mixed RX/TX mode with four antennas comprises several drawbacks. Firstly, the antenna system is forced to operate on separated and widened bands. For example, in the Ka-band, transmission is performed in the 20-GHz band and reception is performed in the 30-GHz band. The antenna must offer a compromise as regards a set of parameters such as size of the spots, the spatial selectivity of the spots (stated otherwise, the roll-off), the C/I level over 150% of the bandwidth, the ratio C/I being the ratio of the useful signal C to the interfering signals I arising from the neighbouring spots. Moreover, this architecture employs complex primary source blocks with broadband sources as well as complex and expensive frequency and polarization extractors. Furthermore, the filtering between the transmit mode and the receive mode may turn out to be critical if the transmit mode is high power. Finally, installation on a satellite leads to double deployments of the reflectors. The storage constraints for these reflectors give rise to interactions of a mechanical kind with the structures of the launcher so that inevitable truncations at the level of the geometry of the reflectors of greater or smaller amplitude occur, as illustrated in FIG. 1, where the reflectors 101, 102, 103, 104 are truncated on two sides. Consequently, an architecture with four reflectors in RX/TX mode causes a rotation of 90° of the coverages at each antenna hop, substantially elliptical coverages, unacceptable C/I levels because of the upswings in the lateral or side lobes and a reduction in the utilizable area of the reflectors of possibly as much as 1 dB or indeed 1.5 dB as regards the accessible maximum directivity. All these factors reduce the capacity of the antenna system.